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May24-08, 09:01 PM
Jorrie's Avatar
P: 765
Quote Quote by tiny-tim View Post
And even light in orbit at exactly 1.5 will be in unstable orbit, and so will either spiral in or spiral out.

But give it a kick radially inwards, from 1.5, and I think it just reaches the opposite point of the orbit with the same kick outwards, and then escapes.
Hi tiny-tim. You're right about the instability, but I think you've got it wrong with the effect of the 'kick'.

For any particle, including light, to move as you described, it must pass a point with pure horizontal (tangential) velocity and with r < 1.5R_hor. Unless it can move at a local speed exceeding c, it will spiral in, don't you think? Look at the 'impact parameter' of MTW.